Optical pickup device with diffraction device

ABSTRACT

An optical pickup device in which a main beam and a pair of sub-beams are used and a diffraction device is disposed between a recording medium and a light receiving device such as a photodetector is disclosed. The diffraction device comprises first to third diffraction regions. The second and third regions receive light beams from the recording medium which are substantially identical in amount to each other. The light receiving device comprises a first to a fourth light receiving regions. The first and second light receiving regions are juxtaposed, and separated by a line. The main beam which has been diffracted by the first diffraction region is focused onto said line. The main beam which has been diffracted by the second diffraction region is focused onto the first light receiving region. The main beam which has been diffracted by the third diffracting region is focused onto the second light receiving region. The sub-beams which have been diffracted by the diffraction device are focused onto the third and fourth light receiving regions, respectively. Alternatively, the diffraction device comprises an area where one or more diffraction regions are formed. The farthest point of the area at the side of the light receiving device is separated from the optical axis by a predetermined distance, to prevent the first order diffracted beam from entering the optical system disposed between the diffraction device and a recording medium.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to an optical pickup device which is useful in aninformation recording and/or reproducing apparatus such as a compactdisc reproducing apparatus, a video disc reproducing apparatus and thelike.

2. Description of the prior art

FIG. 9 shows a conventional optical pickup device. The device of FIG. 9comprises a semiconductor laser device 1, a first diffraction device 2,a second diffraction device 23, a collimating lens 4, an object lens 5,and a photodetector 17. A light beam from the semiconductor laser device1 is diffracted by the first diffraction device 2 to produce threeseparate light beams, one of which is the zero-order diffracted beam(hereinafter, referred to as "the main beam"), and the others of whichare the first-order diffracted beams (hereinafter, referred to as "thesub-beams") in the positive and negative directions which aresubstantially orthogonal to the sheet of the drawing of FIG. 9. Thesethree separate beams are further diffracted by the second diffractiondevice 23. The resulting zero-order diffracted beam of each of theabove-mentioned separate beams enters the object lens 5 through thecollimating lens 4, and is focused on a recording medium 6 The main beamis focused on a pit of the recording medium 6. The two sub-beams, whichare positioned symmetrically with respect to the above-mentioned mainbeam, are focused on the recording medium 6 in such a manner that theyshift to a large extent in the tracking direction of the recordingmedium 6 and to a small extent in the radial direction. The beamsreflected from the recording medium 6 pass through the object lens 5 andthe collimating lens 4, and are diffracted by the second diffractiondevice 23. The resulting first-order diffracted beams are introducedinto the photodetector 17.

As shown in FIG. 10A, the diffraction device 23 is divided into twodiffraction regions 23a and 23b by a division line 23c, when viewed fromthe side of the recording medium 6. The regions 23a and 23b have anumber of grating lines which are inclined with respect to the divisionline 23c and which are symmetrical about the division line 23c. Thephotodetector 17 is divided into six regions 17a to 17f, as shown inFIG. 10B. The division line 23c elongates in the radial direction of therecording medium 6.

When a beam from the semiconductor laser device 1 is precisely focusedon the recording medium 6 or set at the correct focus, the resultingmain beam which has been diffracted by the region 23a of the diffractiondevice 23 is focused on the division line A₁ of the photodetector 17 toform a spot Q₁, and the resulting main beam which has been diffracted bythe region 23b of the diffraction device 23 is focused on the divisionline B₁ to form a spot Q₂. The resulting sub-beams are focused on theregions 17e and 17f of the photodetector 17. When output signals of thephotodetecting regions 17a to 17f are represented respectively as S_(1a)to S_(1f), a focus error signal is obtained by calculating (S_(1a)+S_(1d))-(S_(1b) +S_(1c)), a tracking error signal is obtained bycalculating (S_(1e) -S_(1f)) and a pit signal (i.e., an informationsignal) is obtained by calculating (S_(1a) +S_(1b) +S_(1c) +s_(1d)).

FIG. 11 shows another conventional optical pickup device. The opticalpickup device of FIG. 11 is different from the above-mentionedconventional device in the configurations of a second diffraction device33 and a photodetector 27 The configuration of the grating lines of thediffraction device 33 and that of the photodetector 27, which are viewedfrom the recording medium 6, are shown in FIGS. 12A and 12B,respectively The diffraction device 33 is divided into two regions 33aand 33b by a division line 33c which elongates in the radial direction.The regions 33a and 33b have a number of grating lines which are atright angles to the division line 33c, and the grating period of theregion 33a is different from that of the other region 33b. Thephotodetector 27 is divided into six regions 27a to 27f. When a beamfrom the semiconductor laser device 1 is precisely focused on therecording medium 6 or set at the correct focus, the resulting main beamwhich has been diffracted by the region 33a is focused on the divisionline A₂ to form a spot R₁ thereon, and the resulting main beam which hasbeen diffracted by the region 33b is focused on the division line B₂ toform a spot R₂ thereon. The resulting sub-beams are focused on thephotodetecting regions 27e and 27f. When output signals of thephotodetecting regions 27a to 27f are represented respectively as S_(2a)to S_(2f), a focus error signal is obtained by calculating (S_(2a)+S_(2d))-(S_(2b) +S_(2c)), a tracking error signal is obtained bycalculating (S_(2e) -S_(2f)), and a pit signal is obtained bycalculating (S_(2a) +S_(2b) +S_(2c) +S_(2d)).

In the conventional optical pickup devices with the above-mentionedstructures, the spots Q₁ and Q₂ (R₁ and R₂) based on the beams reflectedfrom the recording medium 6 must be very precisely formed on thedivision lines of the photodetector 17(27). To achieve this, a delicateadjustment must be carried out so that the diffraction device 23(33) andthe photodetector 17(27) respectively can be disposed at a givenposition. However, in order that the diffraction device 23(33) and thephotodetector 17(27) are constructed to be moved separately orindependently from the diffraction device 23(33), there must be asupporting structure by which the photodetector 17(27) can be freelymoved. This makes the entire structure of the pickup device complicated,causing difficulties in obtaining a light-weight, miniaturized pickupdevice. Moreover, a number of positioning parts are needed, which makesthe production process of the pickup device complicated and makes theproduction cost expensive.

To solve these problems, the inventors of this invention designed toincorporate both the semiconductor laser device 1 and the photodetector17(27) into the same package so that the positioning of the spots Q₁ andQ₂ (R₁ and R₂) on the division lines of the photodetector 17/(27) can becarried out by the positional adjustment of the diffraction device23(33) alone. However, in a optical pickup device with such a structure,the slight shifting of the positions of the photodetector 17(27) fromthose of the initial plan makes it impossible to form the beam spots atthe correct positions of the photodetector 17(27), resulting in a focusoffset. To remove this focus offset, the position of the diffractiondevice 23(33) must be moved linearly and/or rotationally with respect toother components such as the semiconductor laser device 1 to shift thespots on the photodetector 17(27), so that the focus error signalbecomes zero when the beam from the semiconductor laser device 1 is atthe correct focus on the recording medium 6. However, the two spots onthe photodetector 17(27) which are formed based on the main beams shiftat the same time, resulting in that the position of each beam spotcannot be independently adjusted without the simultaneous shifting ofthese beam spots on the photodetector 17(27). Moreover, there is apossibility that the shifting of the two spots are countervailed on thefocus error signals corresponding thereto. To avoid this, thediffraction device 23(33) must be moved to a great extent in theY-direction (FIGS. 10B and 12B). Especially, in the optical pickupdevice shown in FIG. 11, the length of each of the divided regions ofthe photodetector 27 in the y-direction is short. Therefore, when agreat focus offset occurs and the diffraction device 33 is moved to agreat extent in the y-direction to compensate the said focus offset, thebeam spots R₁ and R₂ on the photodetector 27 shift to a great extent inthe y-direction and slip out of the photodetecting regions on whichthese spots must be formed.

Moreover, because the diffraction device 23 (33) must be moved linearlyto compensate for the focus offset phenomenon, the photodetector 17(27)is required to have a large enough size to receive the beam spotsthereon, which makes the production cost thereof expensive.

FIG. 13 shows an optical pickup device which is disclosed in U.S. Ser.No. 07/282,109, European Patent Appln. No. 88311665.9 and CanadianPatent Appln No. 585,356. In the pickup device of FIG. 13, thediffraction device 43 consists of two diffraction regions 43a and 43bwhich are formed by dividing the whole area of the diffraction device bya division line 43c. The diffraction regions 43a and 43b have a numberof grating lines which elongate in the track direction of the recordingmedium 6 (X--X' direction in FIG. 13). The division line 43c elongatesin the direction (Y--Y' direction in FIG. 13) which is perpendicular tothe track direction. The photodetector 37 is divided into five regions37a to 37e. The division line 37f extends in the Y--Y' direction toseparate the regions 37a and 37b.

The main beam which has been diffracted by the diffraction region 43a isfocused on the division line 37f to form a spot S₁, and the main beamwhich has been diffracted by the diffraction region 43b is focused onthe region 37c to form a spot S₂. The sub-beams are focused on theregions 37d and 37e to form spots S₃ to S₆. When a beam from thesemiconductor laser device 1 is precisely focused on the recordingmedium 6, the spots S₁ to S₆ are formed as tiny spots as shown in FIG.14B. In contrast, when the distance between the recording medium 6 andthe object lens 5 becomes small (or large), the spots S₃ to S₆ areformed so as to be extended in semicircular shapes as shown in FIG. 14B(or in FIG. 14C). The regions 37a to 37e on

which spots S₃ to S₆ are formed produce signals S_(a) to S_(e),respectively. A focus error signal is obtained by calculating (S_(a)-S_(b)), a tracking error signal by calculating (S_(d) -S_(e)), and aninformation signal by calculating (S_(a) +S_(b) +S_(c)).

The optical pickup device of FIG. 13 has a drawback in that a spurioussignal is generated in the focus signal, tracking error signal andinformation signal. This will be described with reference to FIGS. 15and 16. A light beam emitted from the semiconductor laser device 1 isdivided into a main beam and two sub-beams by the diffraction device 2,and enters in the diffraction device 43. For example, the resultingfirst-order diffracted beam diffracted by dividing the diffractionregion 43b propagates as indicated by the phantom lines A, as if it is alight beam emitted from the spot S₂ toward the diffraction device 43.Therefore, this first-order diffracted beam is focused by the lenses 4and 5 on the position of the recording medium 6, a position whichcorresponds to the spot S₂ (i.e., the position is the image point of thespot S₂). As shown by the phantom lines B, this first-order diffractedbeam is reflected by the recording medium 6 to enter into thediffraction device 43 through the lenses 5 and 4, and the resultingzero-order diffracted beam is focused on the photodetector 37 as thespot S₂. Similarly, the resulting first-order diffracted beam diffractedby dividing the diffraction region 43a is reflected by the recordingmedium 6, and then focused on the photodetector 37 as the spot S₁.

The first-order diffracted beams which are diffracted by the diffractiondevice 43 are not the light beams to be used for detecting signals. Whenthese first-order diffracted beams are once received by thephotodetector 37, therefore, a spurious signal is generated in the focuserror signal and information signal. This causes the focus control to beincorrectly conducted, and an incorrect information signal to beproduced. Also, the diffraction device 43 produces first-orderdiffracted beams based on the sub-beams which have been diffracted bythe diffraction device 2, so that a spurious signal is also generated inthe tracking error signal, thereby impeding the tracking control In thisway, the first-order diffracted beams produced by the diffraction device43 cause a spurious signal in detection signals so that the opticalpickup device cannot function properly.

SUMMARY OF THE INVENTION

The optical pickup device of this invention, which overcomes theabove-discussed and numerous other disadvantages and deficiencies of theprior art, comprises a main beam and a pair of sub-beams are reflectedfrom a recording medium, and directed to a light receiving devicethrough a diffraction device, said diffraction device comprises first tothird diffraction regions, said second and third diffraction regionsreceiving light beams from said recording medium which are substantiallyidentical in amount to each other, said light receiving device comprisesa first to a fourth light receiving regions, said first and second lightreceiving regions being juxtaposed, and separated by a line, the mainbeam which has been diffracted by said first diffraction region isfocused onto said line, the main beam which has been diffracted by saidsecond diffraction region is focused onto said first light receivingregion, the main beam which has been diffracted by said thirddiffraction region is focused onto said second light receiving region,and the sub-beams which have been diffracted by said diffraction deviceare focused onto said third and fourth light receiving regions,respectively.

In a preferred embodiment, the line extends along a direction whichsubstantially coincides with the diffraction direction along which saidmain beam is diffracted in said first diffraction region.

In a preferred embodiment, the diffraction regions are different ingrating period from each other.

In a preferred embodiment, the diffraction device has a circular shape.

In a preferred embodiment, the first diffraction region is formed into asemicircular shape, and said second and third diffracting regions areformed into a quadrant shape, respectively.

In a preferred embodiment, gratings of said first to third diffractionregions are inclined to each other.

In a preferred embodiment, gratings of said second and third diffractionregions extend at opposite angles with respect to said line.

In a preferred embodiment, the first diffraction region is formed into asemicircular shape, said second and third diffracting regions share asemicircle, and said second diffracting region is formed into astrip-like shape positioned between said first and third diffractionregions.

The optical pickup device comprises a light source; a diffractiondevice; an optical system disposed between said diffraction device and arecording medium, said optical system focusing light beams which includeat least first-order diffracted beams and which are diffracted by saiddiffracting device, onto said recording medium, said optical systemdirecting light beams reflected from said recording medium to saiddiffraction device; and a light receiving device which receives lightbeams from said diffraction device, said diffraction device comprises anarea where one or more diffraction regions are formed, the optical axisof said light source passing through said area, the farthest point ofsaid area at the side of said light receiving device being separatedfrom said optical axis by a predetermined distance, the other farthestpoint of said area at the opposite side of said light receiving devicebeing separated from said optical axis by a distance which is greaterthan said predetermined distance.

In a preferred embodiment, a first-order diffracted beam from saiddiffraction device is prevented from entering into said optical system.

In a preferred embodiment, the diffraction device comprises another areawhere no diffraction region is formed, said other area being positionedmore closely to said light receiving device than said area.

Thus, the invention described herein makes possible the objectives of

(1) providing an optical pickup device in which the focus offset can beadjusted simply by rotating a diffraction device;

(2) providing an optical pickup device in which the focus offset can beeasily adjusted without linearly moving a diffraction device;

(3) providing an optical pickup device in which the size of a lightreceiving device can be reduced;

(4) providing an optical pickup device in which a spurious signal is notproduced; and

(5) providing an optical pickup device which can function properly.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 illustrates diagrammatically an optical pickup device accordingto the invention.

FIG. 2A is a plan view of a diffraction vice used in the optical pickupdevice of FIG. 1.

FIG. 2B is a plan view of a photodetector used in the optical pickupdevice of FIG. 1.

FIG. 3 is a plan view of a diffraction device used in another opticalpickup device according to the invention

FIG. 4 is a perspective view illustrating a further optical pickupdevice according to the invention.

FIG. 5 is a section view of the optical pickup device of FIG. 4.

FIG. 6 is a plan view of a diffraction device used in the optical pickupdevice of FIG. 5.

FIGS. 7A, 7B, 7C, and 7D illustrate various modifications of thediffraction device useful in the optical pickup device of FIG. 5.

FIG. 8 illustrates positional relations of spots formed on thediffraction device of FIG. 6.

FIG. 9 illustrates diagrammatically a conventional optical pickupdevice.

FIG. 10A is a plan view of a diffraction device used in the opticalpickup device of FIG. 9.

FIG. 10B is a plan view of a photodetector used in the optical pickupdevice of FIG. 9.

FIG. 11 illustrates diagrammatically another conventional optical pickupdevice.

FIG. 12A is a plan view of a diffraction device used in the opticalpickup device of FIG. 11.

FIG. 12B is a plan view of a photodetector used in the optical pickupdevice of FIG. 11.

FIG. 13 is a perspective view illustrating an improved optical pickupdevice.

FIGS. 14A, 14B, and 14C illustrate positional relations of spots formedon the diffraction device shown in FIG. 13.

FIG. 15 is a section view illustrating the optical pickup device of FIG.13.

FIG. 16 is a plan view of the diffraction device used in the opticalpickup device of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an optical pickup device according to the invention. Thepickup device of FIG. 1 comprises a semiconductor laser device 1, afirst diffraction device 2, a second diffraction device 3, a collimatinglens 4, an object lens 5, and a photodetector 7. In this embodiment, thesemiconductor laser device 1 and photodetector 7 are mounted to a singlebase in the same package so that their positional relationship is fixed.The laser device 1, first diffraction device 2, collimating lens 4 andobject lens 5 can be constructed in a similar manner to those used inthe pickup device of FIG. 9 except for a second diffraction device 3 andphotodetector 7, and therefore their detailed description is omitted.

As shown in FIG. 2A, the diffraction device 3 has a circular shape as awhole, and is divided into three diffraction regions 3a, 3b and 3c bydivision lines 3d and 3e, when viewed from the side of the recordingmedium 6. The division line 3d corresponds to a diameter of the circleso that the first diffraction region 3a is formed into a semicircularshape. The second and third diffraction regions 3b and 3c are formed bydividing the other semicircle by the division line 3e which extends fromthe center of the circle and is perpendicular to the division line 3d,thereby making the two regions 3b and 3c receive a same amount of lightas each other. The grating lines of the second and third diffractionregions 3b and 3c, which are inclined with respect to the grating linesof the first diffraction region 3a, extend at an opposite angle. Inorder to compensate the aberration, in the three regions 3a to 3c, thegrating lines are gently curved, and the grating periods are graduallychanged. In this embodiment, the center grating period of the firstdiffraction region 3a is smaller than that of the second diffractionregion 3b, and greater than that of the third diffraction region 3c. Theorder of the grating periods is not limited to the above, but areadequately determined in accordance with the relative positionalrelationship between the light source 1, the second diffraction device 3and spots on the photodetector 7 which will be described below.

The photodetector 7 is divided into four regions 7a, 7b, 7e and 7f, asshown in FIG. 2B. The regions 7a and 7b are juxtaposed, and separated bythe division line A₀. The division line A₀ elongates in substantiallythe same direction as the diffraction direction but is slightly inclinedwith respect to the diffraction direction, so as to prevent a focusoffset due to the change in wavelength from occurring.

When a beam from the semiconductor laser device 1 is precisely focusedon the recording medium 6, the resulting main beam which has beendiffracted by the diffraction region 3a of the diffraction device 3 isfocused on the division line A₀ of the photodetector 7 to form a spotP₁, the resulting main beam which has been diffracted by the region 3bis focused on the region 7a to form a spot P₂, and the resulting mainbeam which has been diffracted by the region 3c is focused on the region7b to form a spot P₃. The resulting sub-beams which have been diffractedby the diffraction device 3 are focused on the regions 7e and 7f or thephotodetector 7. When output signals of the photodetecting regions 7a,7b, 7e and 7f are represented respectively as S_(a), S_(b), S_(e) andS_(f), a focus error signal is obtained by calculating (S_(a-S) _(b))(the single knife edge method), a tracking error signal is obtained bycalculating (S_(e) -S_(f)) (the three-beam method), and a pit signal isobtained by calculating (S_(a) +S_(b)).

In a pickup device having the above-described structure, the intensityof the spot P₂ equals to that of the spot P₃ so that they are offset inthe focus error signal (S_(a) -S_(b)). Therefore, the focus offset canbe adjusted simply by moving the spot P₁ formed on the photodetector 7in the x-direction (FIG. 2B). This movement of the spot P₁ can beperformed by rotating the diffraction device 3. In this way, the focusoffset can be adjusted without linearly moving the diffraction device 3.Therefore, the size of the diffraction device 3 can be reduced so thatit equals the size of a light beam entering thereon, thereby reducingthe manufacturing cost and the size of the optical pickup device.

According to the invention, the diffraction device 3 may be constructedas shown in FIG. 3. The diffraction device 3 of FIG. 3 is divided intothree diffraction regions 8a, 8b and 8c by two division lines 8d and 8e,when viewed from the side of the recording medium 6. The division line8d corresponds to a diameter of the circle, and the first diffractionregion 8a is formed into a semicircular shape. The division line 8e runsin the remaining semicircle in parallel with the division line 8d, sothat the two regions 8b and 8c receive a same amount of light as eachother. The grating lines of the second and third diffraction regions 8band 8c which are inclined with respect to the grating line of the firstdiffraction region 8a, extend at an opposite angle. In order tocompensate the aberration, in the three regions 8a to 8c, the gratinglines are gently curved, and the grating periods are gradually changed.In this embodiment, the center grating period of the first diffractionregion 8a is smaller than that of the second diffraction region 8b, andgreater than that of the third diffraction region 8c. For the reasonmentioned above, the order of the grating periods is not limited to theabove, but may be adequately determined.

In the embodiment using the diffraction device 3 of FIG. 3, the lightbeams diffracted by the diffraction device 3 form spots on thephotodetector 7 of FIG. 2B in the same manner as the pickup device ofFIG. 1, and therefore the focus offset can be adjusted simply by movingthe spot P₁.

FIG. 4 shows a further optical pickup device according to the invention.The pickup device of FIG. 4 is constructed in the same manner as thepickup device of FIG. 13 except for a second diffraction device 13. Inthis embodiment, the second diffraction device 13 has a square shapewhen viewed from the recording medium 6, and is divided into tworectangular areas 13e and 13f which are separated by a division line13d. The division line 13d, which is not an actually formed one but animaginary one for describing the area 13e, runs in the track direction(X--X' direction) of the recording medium 6, and is separated from theoptical axis C of a light beam emitted from the semiconductor laserdevice 1, by a predetermined distance L toward the photodetector 27. Thedistance between the optical axis and the farthest point of the area 3eat the opposite side of the photodetector 27 is greater than thedistance L. The area 13e is divided by a division line 13c which isperpendicular to the division line 13d (i.e., the division line 13c runsin the Y--Y' direction), into two rectangular diffraction regions 13aand 13b where grating lines are formed in the track direction. Thegrating period of the first diffraction region 13a is slightly smallerthan that of the second diffraction region 13b. The value of thedistance L is selected so that the resulting first-order diffractedbeams diffracted by the diffraction regions 13a and 13b do not enter thecollimating lens 4.

In the pickup device of FIG. 4, the diffracted beams diffracted by thefirst diffraction device 2 are further diffracted by the seconddiffraction device 13. Because the second diffraction device 13 isprovided with gratings only in the area 13e, the diffraction directionof each of the first-order diffracted beams is restricted as shown bythe phantom lines A in FIG. 5 so that the first-order diffracted beamsare prevented from entering the collimating lens 4. That is, thefirst-order diffracted beams diffracted by the diffraction device 13 arenot focused, and therefore the photodetector 27 is substantiallyprevented from receiving the first-order diffracted beams reflected fromthe recording medium 6, thereby effectively suppressing the level of aspurious signal generated in the focus error signal, tracking errorsignal and information signal.

In the above-mentioned embodiment, gratings are formed in the whole ofthe area 13e of the diffraction device 13. The structure of thediffraction device 13 is not restricted to the above, but can beadequately formed in another manner. For example, the gratings areformed in a rectangular shape which is positioned at the center of thediffraction device 13 and elongates along the X--X' direction (FIG. 7A),in a rectangular shape which is shorter than the diffraction device 13in the X--X' direction (FIG. 7B), in an oval shape (FIG. 7C), or in ashape which is a partly removed circle (FIG. 7D). The range of formingthe diffraction ranges 13a and 13b my be restricted in either of theX--X' direction or the Y--Y' direction. When the diffraction device 13is used in the three-beam method as in the case of this embodiment, themain beam (solid line) and sub-beams (two-dot dash lines) reflected fromthe recording medium 6 are slightly shifted from each other, as shown inFIG. 8. Hence, it is preferable that the diffraction ranges 13a and 13bare formed wider in the X--X' direction.

Moreover, the area 13f of the diffraction device 13 may be omitted asrequired.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. In an optical pickup device wherein a main beamand a pair of sub-beams are reflected from a recording medium, anddirected to a light receiving device through a diffraction device, theimprovement wherein:said diffraction device comprises first, second andthird diffraction regions which diffract the main beams and sub-beamsalong predetermined diffraction directions, said second and thirddiffraction regions receiving light beams from said recording mediumwhich are substantially identical in amount to each other, said lightreceiving device comprises first, second, third and fourth receivingregions, said first and second light receiving regions being juxtaposed,and separated by a line, the main beam which has been diffracted by saidfirst diffraction region is focused onto said line, the main beam whichhas been diffracted by said second diffraction region is focused ontosaid first light receiving region, the main beam which has beendiffracted by said third diffraction region is focused onto said secondlight receiving region, and the sub-beams which have been diffracted bysaid diffraction device are focused onto said third and fourth lightreceiving regions, respectively.
 2. An optical pickup device accordingto claim 1, wherein said line extends along a direction whichsubstantially coincides with the diffraction direction along which saidmain beam is diffracted in said first diffraction region.
 3. An opticalpickup device according to claim 1, wherein said diffraction regions aredifferent in grating period from each other.
 4. An optical pickup deviceaccording to claim 1, wherein said diffraction device has a circularshape.
 5. An optical pickup device according to claim 4, wherein saidfirst diffraction region is formed into a semicircular shape, and saidsecond and third diffracting regions are formed into a quadrant shape,respectively.
 6. An optical pickup device according to claim 1, whereingratings of said first, second and third diffraction gratings are notparallel to each other.
 7. An optical pickup device according to claim6, wherein gratings of said second and third diffraction regions extendat opposite angles with respect to said line.
 8. An optical pickupdevice according to claim 4, wherein said first diffraction region isformed into a semicircular shape, said second and third diffractingregions share a semicircle, and said second diffracting region is formedinto a strip-like shape positioned between said first and thirddiffraction regions.
 9. In an optical pickup device comprising: a lightsource; a diffraction device; an optical system disposed between saiddiffraction device and a recording medium, said optical system focusinglight beams from said light source onto said recording medium, saidoptical system directing light beams reflected from said recordingmedium to said diffraction device; and a light receiving device which isspaced transversely from the optical axis of the device and receiveslight beams from said diffraction device, the improvement wherein:saiddiffraction device comprises an area where one or more diffractionregions are formed, said area having a first side and a second side withthe first side being transversely nearer the receiving device than thesecond side, the first side of said area being separated from saidoptical axis by a predetermined distance, and the second side beingseparated from said optical axis by a distance which is greater thansaid predetermined distance. .Iadd.
 10. An optical pickup devicecomprising:a light source for emitting light beams; an optical systemfor focusing said light beams emitted from said light source onto arecording medium, said optical system allowing said light beamsreflected from said recording medium to pass therethrough; a lightreceiving means which receives said light beams reflected from saidrecording medium; a diffraction means, disposed between said lightsource and said optical system across an optical axis of dividing saidlight beams emitted from said light source into at least a zero-orderdiffracted beam and first-order diffracted beams and for introducingsaid light beams reflected from said recording medium into said lightreceiving means; wherein said diffraction means has a diffraction areawhere a diffraction grating is formed, said optical axis of said lightbeams emitted from said light source passing through a position of saiddiffraction area, said position being separated from an end portion ofsaid diffraction area by a predetermined distance, said end portionbeing closer to said light receiving means than any other end portion ofsaid diffraction area; and wherein said predetermined distance is set sothat said first-order diffracted beams from the diffraction means arenot incident on said optical system..Iaddend..Iadd.11. A optical pickupdevice according to claim 10, wherein tracks are formed on saidrecording medium, and said diffraction area is formed so that a widththereof in a direction parallel to said tracks is larger than a width ofsaid diffraction area in a direction perpendicular to saidtracks..Iaddend..Iadd.12. An optical pickup device comprising: a lightsource for emitting light beams; an optical system for focusing saidlight beams emitted from said light source onto a recording medium, saidoptical system allowing said light beams reflected from said recordingmedium to pass therethrough; a light receiving means which receives saidlight beams reflected from said recording medium; a diffraction means,disposed between said light source and said optical system across anoptical axis of said light beams emitted from said light source, fordividing said light beams emitted from said light source into at least azero-order diffracted beam and a plurality of first-order diffractedbeams and for introducing said light beams reflected from said recordingmedium into said light receiving means; wherein said diffraction meanshas a diffraction area where a diffraction grating is formed, saidoptical axis of said light beams emitted from said light source passingthrough said diffraction area; and wherein a distance from said opticalaxis to an end portion of said diffraction area is set so that saidfirst-order diffracted beams from said diffraction means are notincident on said optical system..Iaddend..Iadd.13. An optical pickupdevice comprising: a light source for emitting light beams; an opticalsystem for focusing said light beams emitted from said light source ontoa recording medium, said optical system allowing said light beamsreflected from said recording medium to pass therethrough; a lightreceiving means which receives said light beams reflected from saidrecording medium; a diffraction means, disposed between said lightsource and said optical system across an optical axis of said lightbeams emitted from said light source, for dividing said light beamsemitted from said light source into at least a zero-order diffractedbeam and a plurality of first-order diffracted beams and introducingsaid light beams reflected from said recording medium into said lightreceiving means; wherein said diffraction means has a diffraction areawhere a diffraction grating is formed, said diffraction area having afirst end portion and a second end portion in a direction perpendicularto said diffraction grating, a distance between said second end portionand said light receiving means being larger than a distance between saidfirst end portion and said light receiving means; and wherein saidoptical axis of said light beams emitted from said light source passesthrough a position of said diffraction area at a predetermined distancefrom said first end portion; and wherein said predetermined distance isset so that said first-order diffracted beams from said diffractionmeans are not incident on said optical system..Iaddend..Iadd.14. Anoptical pickup device according to claim 13, wherein a width of saiddiffraction area in a direction parallel to said diffraction grating islarger than a width of said diffraction area in a directionperpendicular to said diffraction grating..Iaddend.